Display screen or portion thereof with transitional graphical user interface



FIG. 1 is the first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing my new design; and,

FIG. 2 is the second image thereof.

The dash-dash broken lines around the figure show a display screen or portion thereof, which forms no part of the claimed design; the dash-dash broken lines within the figure show unclaimed portions for the graphical user interface. The dot-dash broken lines within the figure represent an unclaimed boundary line that forms no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-2. The process or period in which one image transitions to another image forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described. 